1. Field of the Invention
The present invention relates to an internal combustion engine provided with a centrifugal decompressing means for reducing compression pressure to facilitate staring the internal combustion engine by opening a valve included in the internal combustion engine during the compression stroke in starting the internal combustion engine, and a method of adjusting valve lift for decompression.
2. Description of the Related Art
Internal combustion engines provided with a centrifugal decompressing means including a flyweight are disclosed in JP2001-221023A and JP63-246404A. A decompression member included in the decompressing means disclosed in JP2001-221023A or JP63-246404A is a plate-shaped member of a substantially uniform thickness integrally provided with a flyweight and a decompression cam. A support pin supporting the flyweight for swing motion is extended through a middle part of a camshaft substantially perpendicularly to the axis of the camshaft. It is difficult to form the camshaft in a lightweight, hollow member and to form an oil passage through the camshaft when the support pin supporting the flyweight of the decompressing means is extended through the camshaft substantially perpendicularly to the axis of rotation of the camshaft.
An internal combustion engine proposed in JP11-294130A is provided with a decompressing means including a flyweight supported for swing motion by a pin on a camshaft provided with a central oil passage. This prior art internal combustion engine has a camshaft provided with a cam held in contact with a valve tappet, and a central oil passage; and a compressing means including a decompression member having the shape of a plate of a substantially uniform thickness and a function of a flyweight, and a return spring. The decompression member is provided with a protrusion, which corresponds to a decompression cam, formed integrally with a flyweight. The protrusion lifts up the valve tappet in a starting phase of the internal combustion engine to open an exhaust valve. The decompression member is supported for swing motion by a pair of pins placed on the camshaft at positions deviated from the central part provided with the oil passage of the camshaft.
In the decompressing means disclosed in JP11-294130A, the pair of pins are disposed on a diameter of the camshaft, the axis of turning of the decompression member, similarly to those of the decompressing means disclosed in JP2001-221023A and JP63-246404A, is substantially perpendicular to the axis of rotation of the camshaft. Therefore, it is difficult to secure a space in which a fully expanded flyweight included in the decompressing means revolves about the axis of rotation of the camshaft, i.e., to narrow a cylindrical space in which a fully expanded flyweight included in the decompressing means revolves about the axis of rotation of the camshaft, and hence a comparatively large space must be secured for the decompressing means around the camshaft, which increases the size of the internal combustion engine. For example, it is difficult for the prior art supposed to extend the center axis of turning substantially perpendicularly to the axis of rotation of the camshaft to narrow the space necessary for the revolution of the fully expanded decompression member because the prior art needs a long distance between the center axis of swing motion and a position where the cam is in contact with a cam follower, such as a valve tappet or a rocker arm. The wall thickness of the camshaft provided with the central oil passage of the internal combustion engine disclosed in JP11-294130A must be greater than the depth of a hole in which the pin is fitted, and hence the diameter of the oil passage is limited and the oil passage must be formed in a comparatively small diameter.
When the weight of the decompression member is reduced to reduce the weight of the internal combustion engine, it is preferable to increase the distance between the position of the center of gravity of the decompression member at an initial position from which the decompression member starts swinging and the axis of rotation of the camshaft to ensure that a necessary centrifugal force is produced at a predetermined engine speed at which a decompressing operation is stopped. However, the decompressing means disclosed in JP2001-221023A and JP63-246404A need to increase the length of the decompression member to increase the distance between the center of gravity of the decompression member and the axis of rotation of the camshaft, which, sometimes, increases the diameter of a cylindrical space necessary for the fully expanded decompression member to turn around the camshaft.
When the distance between the center of gravity of the decompression member and the axis of rotation of the camshaft is increased in the prior art decompressing means including the plate-shaped decompression member of a substantially uniform thickness, not only the size of the flyweight but also the size of the decompression member must be increased and, eventually, the cylindrical space around the camshaft occupied by the fully expanded decompression member expands. If increase in the size of the decompression member is avoided, additional working steps such as bending a plate, increases inevitably to form the flyweight having the shape of a plate of a substantially uniform thickness such that the weight is concentrated on the flyweight, the flyweight has a complicated shape that requires difficult machining, the difference in operating characteristic between different decompression member increases.
The present invention has been made in view of the foregoing problems and it is therefore an object of the present invention to reduce the diameter of a cylindrical space around a camshaft in which a fully expanded decompression member revolves.
Another object of the present invention is to form a decompressing means in a comparatively small size, facilitating securing a necessary mass for a flyweight, facilitating manufacturing decompressing means respectively having operating characteristics distributed in a narrow range and to suppress noise generation due to collision between a flyweight and a camshaft, by changing the thickness of a component member of the decompressing means.